The present invention relates to thermotropic polyesters. More specifically, it relates to copolymers and terpolymers which exhibit optical anisotropy in the molten state.
Liquid crystalline polyesters have several advantages over polyesters which have isotropic melts. Liquid crystalline polymer melts have greatly reduced viscosities when compared to the melts of similar polymers of comparable molecular weights which are isotropic. As a result, they are more easily fabricated from the melt. Also, melts of liquid crystalline polymers orient much more readily during flow than do melts of isotropic polymers. Advantage can be taken of this facile molecular orientation to produce articles with exceptionally high mechanical strengths and stiffness.
However, those molecular features which lead to liquid crystallinity often also result in extremely high melting points. For example, poly(p-oxybenzoate), poly(p-phenylene terephthalate), and poly(p-phenylene naphthalene-2,6-dicarboxylate) all have high melting points of well over 500.degree. C. or degrade before melting. Such high melting points are disadvantageous in that the processing of high-melting polymers is relatively more difficult and costly as compared to the processing of relatively lower-melting polymers.
It is generally known that polyesters can be prepared from aromatic dicarboxylic acids and aromatic diols. For example, European Patent application No. 93,891 discloses polyesters prepared from mixtures of biphenols such as 4,4'-dihydroxy-3,3',5,5'-tetramethyl biphenyl, and dicarboxylic acid derivatives, such as terephthaloyl dichloride optionally with isophthaloyl dichloride. The examples of European Patent application No. 93,891 disclose polymers having Tg's of from 304.degree. to 332.degree. C. H. G. Weyland et al., in European Polymer Journal, 6, 1339-1346 (1970) disclose polyesters prepared from various diacids and biphenols, including 4,4'-dihydroxy-3,3',5,5'-tetramethyl biphenyl. At page 1343 Weyland indicates that methyl or phenyl groups positioned adjacent to the oxygen atoms of the biphenols increase the Tg of the polymers relative to polymers having hydrogen atoms in the same positions, i.e., replacing certain biphenol hydrogen atoms with methyl or phenyl moieties results in an increased Tg. Weyland further indicates that the melting point, Tm, follows the approximate relationship Tg/Tm=2/3.
It is also known to prepare polyesters from aromatic diols, aromatic diacids, and hydroxybenzoates, as taught in U.S. Pat. Nos. 3,975,487 and 3,637,595 to Cottis. The latter patent broadly teaches that the aromatic rings of these polymers preferably are unsubstituted, but can have one to three substituents, such as Cl, Br, F, lower alkyl and lower alkoxy, which do not materially alter the physical and chemical characteristics of the polyesters described therein.
U.S. Pat. No. 4,118,372 is somewhat similar to the two Cottis patents, but broadly discloses that certain aromatic rings can be substituted with one or more Cl, Br, F or C.sub.1-4 alkyl moieties. The disclosed polymers display optical anisotropy in the molten state, and preferably have flow temperatures of from 200.degree. C. to 375.degree. C.
Heretofore, a liquid crystalline polyester having a relatively low melting point has not been prepared using a tetraalkyl biphenol having halogen substitution in the meta position of the aromatic ring(s) relative to the hydroxyl groups.